Does anyone pay attention to Euro Ncap when buying cars?

[kalniel]

Stability control is frankly amazing for the majority of drivers. The effect of getting you around a corner when you are sure you are understeering off the road is incredible.

How does 4x4 give you more traction off the power? You might have smoother braking on all four wheels if you use the engine compared to non-ABS systems, but ABS and stability control provides much more stable breaking than engine breaking these days.

[Konan555]

How does 4x4 give you more traction off the power? You might have smoother braking on all four wheels if you use the engine compared to non-ABS systems, but ABS and stability control provides much more stable breaking than engine breaking these days.

Firstly, keep in mind I said it provided more than RWD. Also, I wasn't comparing it to gadget driven stuff.

I should've phrased it better, it provides more traction to driven wheels.

If you dive into a corner too fast and start to under steer, you can correct it using the brake to operate a weight transfer to the front wheels, causing them to bed down and increase their grip. In an FWD car, so long as you don't get into lift off over steer, this is all well and good. In a RWD situation, you've unloaded the drive wheels and thing's arn't so simple any more.

This is part of why FWD are considered generally safer. The natural reaction to problems is to brake harder or lift off the throttle, which are conducive to bringing a FWD car back in line. Most people don't naturally increase the throttle, flick the wheel and get ready to put some opposite lock back on.

With a 4x4 system, you can simplify it's characteristics to it behaving like a FWD car into corners and a RWD out. If you've gone too fast into a corner you can brake late and if you go too fast out, you can power slide it.

However, on the road none of this is really a good idea... I'm sure it's stability control all the way.

You also mention ABS. ABS on a permanent 4x4 like subaru use is pretty much esential as the 4x4 system works in reverse aswell. Lock up a single wheel and it transmits the lock up to all the others. Not fun.

[5lab]

You also mention ABS. ABS on a permanent 4x4 like subaru use is pretty much esential as the 4x4 system works in reverse aswell. Lock up a single wheel and it transmits the lock up to all the others. Not fun.

only if it had a 4 way diff-lock, which is very unusual on a road-going car (well, some of the fancier 4x4s have it, but its all switchable afaik)

in any case, you'd have to put 4x the braking force onto 1 wheel to make it lock, if there was a locked diff in place (i think??) - it'd be an interesting way to stop a pair of wheels locking

[Konan555]

In the case of subaru, it's not all of them. There's no LSD on the front.

You just need one wheels worth of brakes to make it lock, it's only once there's a significant difference in speed between the wheels the LSDs kick in and lock them together. You also need an LSD to avoid excess stress in the drive train.

[kalniel]

If you dive into a corner too fast and start to under steer, you can correct it using the brake to operate a weight transfer to the front wheels, causing them to bed down and increase their grip. In an FWD car, so long as you don't get into lift off over steer, this is all well and good. In a RWD situation, you've unloaded the drive wheels and thing's arn't so simple any more.

This is part of why FWD are considered generally safer. The natural reaction to problems is to brake harder or lift off the throttle, which are conducive to bringing a FWD car back in line.

You seem to be suggesting braking while the throttle is still open :/ Unloading the drive wheels is no problem when you aren't using them to drive (as in, off the throttle). As long as you aren't jumping on the throttle again, and instead smoothly apply it when you are no longer breaking then the weight transfer settles back nicely on the rear again, which is why RWD cards actually have almost as much grip as 4WD under normal conditions (the front wheels of a 4WD car aren't providing all that much grip if the weight has transfered backwards )

Oversteer caused by lifting off the throttle (as some idiot proved infront of me last night botching a mini roundabout) occurs equally no matter what drive you have - it's just a simple fact of weight transfer. The difference between cars is more to do with how their suspension and importantly diff is set up. FWD cars are basically set up deliberately to understeer. RWD cars tend to be sportier models and have a more neutral setup.

Of course, getting back on the throttle is a big cause of accidents for RWD cars, but the lose of grip while cornering is the same no matter whether it's front or rear: If a car is cornering there is a centripedal force vector that's taking up a certain (large) portion of your grip quota. Any additional forces have to share that same limited quota, which is why you loose grip accelerating while cornering compared to accelerating while going straight.
In fact, as mentioned, there is slightly more grip available to a RWD car because the weight goes back as you start to accelerate.

The problem though is simply that loosing grip at the front has little to no consequence. But loosing grip at the back has a large consequence.

With a 4x4 system, you can simplify it's characteristics to it behaving like a FWD car into corners and a RWD out. If you've gone too fast into a corner you can brake late and if you go too fast out, you can power slide it.

I don't agree - a FWD/RWD/4x4 car all behave identically into a corner unless you are being wierd and keeping the throttle down while braking. Out of a corner a 50:50 front:rear power split 4x4 will behave like a FWD car with a bit more grip. A bigger split in favour of the rear will start behaving more neutrally and thus have more grip, but still it will have only a minor advantage over RWD, because actually a bigger portion of the front wheels grip quota is taken up with the cornering force than the rear wheels, so you might actually only be able to use 10% of your power through the front wheels. But even then, that's grip quota which could probably be better used in cornering rather than accelerating.

You don't want to lock the diffs on a 4x4 on any surface that's got grip - it's a sure way to destroy your tyres and give the car abuse. Even on serious off-roaders it's a last resort and the procedure for locking it is usually quite involved.

[5lab]

In the case of subaru, it's not all of them. There's no LSD on the front.

You just need one wheels worth of brakes to make it lock, it's only once there's a significant difference in speed between the wheels the LSDs kick in and lock them together. You also need an LSD to avoid excess stress in the drive train.

no, you need an lsd or a standard diff. a standard diff allows unlimited slip - i'm sure subarus have a standard diff on the front, which means the spinning of the wheels isnt linked at all.. i'd be surprised if it was a locked diff (cos steerings a bugger with that)

as for fwd vs rwd - i think fwd cars are more likely to understeer cos the weight is generally further fowards of the wheelbase, in order to give grip to the wheels that need it. this means that if you lift off, the weight is doubly-far-forwards, causing more oversteer.. ie, a standard fwd car has a (say) 70:30 front/rear weight distro. under lift off, this transfers to (say) 80:20 - a very light backend liable to spin. a rwd car is more typically 50:50, which under braking transfers to 60:40, thus less lift-off..

i could, naturally, be talking out of my arse.

[Konan555]

Fairly simple explanation to be foud here. http://www.rallycars.com/Cars/4wd_turbo_cars1.html

You'll also need to remember that 4x4s split torque and transfer it when slip occurs to the wheels that arn't slipping.

You don't want to lock the diffs on a 4x4 on any surface that's got grip - it's a sure way to destroy your tyres and give the car abuse. Even on serious off-roaders it's a last resort and the procedure for locking it is usually quite involved.

Which is why they do this super clever thing of only locking when there's wheel slip. Fantastic isn't it.

[Konan555]

no, you need an lsd or a standard diff. a standard diff allows unlimited slip - i'm sure subarus have a standard diff on the front, which means the spinning of the wheels isnt linked at all.. i'd be surprised if it was a locked diff (cos steerings a bugger with that)

Yeah, they run open front, LSD rear and drive the front wheels via a viscous (a la autobox).

If you lock the back right, for example, the viscous will lock together the back left. Once that axle's stopped spinning the viscous in the transfer box will lock and attempt to stop the input shaft to the front diff which will attempt to lock at least one of the front wheels on the open diff. Of course, it might not lock it, it might just roger the viscous coupling. Either way, it aint good.

Works the same in reverse. If you jack the two rear wheels up and one of the fronts, leave the other on the groud, it won't drive that one front (where an audi quattro does).

[5lab]

it wont instantly lock the other rear - it'd make it more likely to happen, granted - theres extra braking force on the wheel, but at the same time it makes it harder to lock any of the wheels - as the diff is using the turning force to help stop the wheels locking

with abs its a little irrelevent

[kalniel]

Fairly simple explanation to be foud here. http://www.rallycars.com/Cars/4wd_turbo_cars1.html

Which backs up what we were saying about corner entry being all about weight distribution and car setup. It's only under power that there's any difference for the same set of parameters. And in my opinion a corner that you don't have to brake to enter isn't really a corner

There's a nice set of articles here about the physics of cornering:

http://phors.locost7.info/contents.htm